The present invention relates to an electrode foil used for an aluminum electrolytic capacitor, and particularly concerns an electrode foil for an aluminum electrolytic capacitor for a high and medium voltage and a method of manufacturing the same.
In recent years, as electronic equipment has become smaller with higher reliability, users have strongly demanded smaller aluminum electrolytic capacitors. Thus, electrode foil used for aluminum electrolytic capacitors needs to be larger in electrostatic capacitance per unit area than the conventional art.
A typical aluminum electrolytic capacitor is configured such that a capacitor element is composed of an anode foil and a cathode foil that are wound via a separator, the capacitor element is dipped into an electrolytic solution for driving, and the capacitor element is sealed into a metallic case. The anode foil has a dielectric oxide film formed by performing anodic oxidation on a surface of aluminum foil, which has an effective surface area increased by etching. The cathode foil includes aluminum foil whose effective surface area is increased by etching.
Regarding such an aluminum electrolytic capacitor, in order to have a larger electrostatic capacitance or a smaller size, it has been necessary to increase an effective surface area of the anode foil and an electrostatic capacitance per unit area. Thus, etching technique for increasing the effective surface area of the anode foil has been developed in earnest.
The above method of etching the anode foil is performed chemically or electrochemically in a solution of hydrochloric acid, in which acid such as sulfuric acid, nitric acid, phosphoric acid, and oxalic acid for forming a film is added. A method of etching the anode foil used for a medium and high voltage basically includes a first etching step of generating main pits and a final etching step of increasing the main pits in a diameter suitable for a used voltage of an aluminum electrolytic capacitor. An important point is how to generate a large number of main pits and to efficiently increase the main pits in size.
For example, as disclosed in JP7-272983A, a technique for increasing an effective surface area of the anode foil includes a first etching step of electrochemically performing etching using a direct current in a solution of hydrochloric acid or the like, an middle etching step of performing etching using a direct current in a solution of neutral salt such as sodium chloride, and a final etching step of performing electrical etching in a solution of nitric acid, sulfuric acid, or mixed acid thereof. With the above manufacturing method, a large number of main pits can be formed from a surface and an effective surface area on aluminum foil can be increased by forming branched sub pits at the midpoints or the ends of the main pits.
Further, a technique disclosed in JP60-36700A includes a first preliminary corrosion step using acid and a second anodizing step of treatment using a direct current with a high current density. Aluminum foil is subjected to an alternating current (AC) treatment and is corroded in the first preliminary corrosion step, and the aluminum foil is subjected to a direct current (DC) treatment in the second anodizing step, so that the aluminum foil is increased in electrostatic capacitance and mechanical strength.
However, according to the above-mentioned technique disclosed in JP7-272983A, in the middle etching step, an etching solution is used which includes at least one of a solution of neutral salt and a solution of acid salt solution. The solution of neutral salt contains at least one of three kinds of chlorine ions including sodium chloride, ammonium chloride, and potassium chloride. When DC etching is performed by using direct current in such an etching solution, in the case where direct current is simply supplied for a fixed time, a large amount of aluminum hydroxide gel is generated around the front ends of etching pits, and sub pits are formed in a vertical direction only on surfaces of the main pits formed in the first etching step. Hence, the above method hardly increases an effective surface area of aluminum foil.
Further, according to the technique disclosed in JP60-36700A, after aluminum foil is subjected to alternating current (AC) treatment and is corroded in the first preliminary corrosion step, the aluminum foil is subjected to a direct current (DC) treatment in the second anodizing step. In this case, although corrosion is surely accelerated, etching pits generated by the direct current treatment have uneven shapes. Thus, it is not possible to obtain a satisfactory electrostatic capacitance and mechanical strength.
Furthermore, according to the conventional technique for increasing an effective surface area of anode foil, in the middle etching step of forming the sub pits, which are branched from the surface layer to the ends of the main pits, on the main pits formed in the first etching step, as shown in FIG. 9, although a surface of aluminum foil 63 is not melted, many sub pits 62 branched from main pits 61 are formed on a surface layer of the aluminum foil 63. Thus, the anode foil cannot be increased in electrostatic capacitance. Here, FIG. 9 is a schematic diagram showing a cross section of etching pits formed by etching on the conventional aluminum foil 63.
The present invention is achieved to solve the above-mentioned conventional problems and has as its object the provision of an electrode foil for an aluminum electrolytic capacitor and a method of manufacturing the same, by which sub pits branched on a surface layer of an aluminum foil can be reduced with high mechanical strength and large electrostatic capacitance.
In order to attain the above object, the electrode foil for an aluminum electrolytic capacitor of the present invention is configured such that a large number of main pits are formed by etching from a surface of aluminum foil in a thickness direction on both surfaces of aluminum foil, and sub pits are branched from the vicinity of a surface layer other than the surface layer on the main pits to the ends of the main pits. With this configuration, sub pits branched on the surface layer of aluminum foil are not formed in the present invention, thereby increasing an electrostatic capacitance of the electrode foil for an aluminum electrolytic capacitor.
Further, in the electrode foil for an aluminum electrolytic capacitor of the present invention, the sub pits are shorter than the main pits. Such a configuration can increase mechanical strength of the electrode foil for an aluminum electrolytic capacitor of the present invention.
The method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention comprises a first etching step in which aluminum foil is dipped into an etching solution of an acidic aqueous solution containing hydrochloric acid and sulfuric acid and/or nitric acid and a direct current is supplied to form main pits, an middle etching step in which the direct current is supplied and etching is performed in an etching solution of neutral salt containing an additive therein to effectively form the sub pits branched from the midpoints or the ends of the main pits other than the surface layer on the main pits, and a final etching step of increasing the main pits and the sub pits in diameter. With this method, since the surface of the aluminum foil is covered with an oxide film, a large number of sub pits are formed in a depth direction of the main pits without forming the sub pits branched into the surface layer of the aluminum foil. Thus, it is possible to produce electrode foil for an aluminum electrolytic capacitor with high mechanical strength and a large electrostatic capacitance in a stable manner.
According to a method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention, in the above-mentioned manufacturing method, at least one or more additives are selected from oxalic acid, citric acid, phosphoric acid, boric acid, succinic acid, and malonic acid. Further, in the manufacturing method of the present invention, a concentration of the additive ranges from 0.01 to 1.0%. With this method, it is possible to obtain the effect of the manufacturing method more effectively.
According to a method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention, in the middle etching step of the above-mentioned manufacturing method, when a direct current is supplied and etching is performed, DC etching is performed while aluminum foil is passed through pairs of electrode plates in a plurality of etching tanks having a plurality of pairs of electrode plates, and a pair of electrode plates for supplying an alternating current is provided to perform AC etching at an upper or lower position of at least a pair of electrode plates among the plurality of electrode plates provided in the plurality of etching tanks. With this method, the surfaces of aluminum foil and etching pits are made rough by supplying an alternating current and a hydrated film is formed. Thus, it is possible to efficiently accelerate the formation of etching pits by using a direct current and to increase an effective surface area of aluminum foil.
According to a method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention, in the above-mentioned manufacturing method, a pair of electrode plates for supplying a direct current is always provided right after a pair of electrode plates for supplying an alternating current. According to the present invention, etching pits can be formed by using a direct current more efficiently than the effect of the above-mentioned manufacturing method.
According to a method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention, in the above-mentioned manufacturing method, a pair of electrode plates for supplying an alternating current partially interrupts a pair of electrode plates for supplying a direct current. The present invention can readily provide a pair of electrode plates for supplying an alternating current.
According to a method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention, in the above-mentioned manufacturing method, a pair of electrode plates for supplying an alternating current is 0.01 to 0.15 A/cm2 in current density. The present invention can enhance the effects of the above-mentioned manufacturing method.
Additionally, when a pair of electrode plates for supplying an alternating current is less than 0.01 A/cm2 in current density, it is not possible to make rough a surface of aluminum foil to form a hydrated film. When a current density exceeds 0.15 A/cm2, the surface becomes too rough and a hydrated film is less likely to be formed.
According to a method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention, in the above-mentioned manufacturing method, in the first etching step and/or final etching step, etching is performed by supplying a direct current as follows: DC etching is performed while aluminum foil is passed through a pair of electrode plates in a plurality of etching tanks having a plurality of pairs of electrode plates, and DC etching is performed while an electrical insulating material partially interrupts at least a pair of electrode plates among the plurality of pairs of electrode plates provided in the plurality of etching tanks. The present invention can obtain a uniform current density in an electrolytic solution, so that etching pits formed on aluminum foil are equal in length. Further, it is possible to increase etching efficiency, thereby achieving a larger effective surface area of aluminum foil.
According to a method of manufacturing the electrode foil for an aluminum electrolytic capacitor of the present invention, in the above-mentioned manufacturing method, an electrical insulating material includes an opening composed of a plurality of holes or a plurality of slits. Hence, it is possible to obtain a more uniform current density in an electrolytic solution.